Control electrode-anode structure for gas discharge devices



1X! .VTO R. [an/A R0 0. Jam/50m E. O. JOHNSON CONTROL ELECTRODE-ANODE 'STRUCTURE Filed Oct. 25, 1952 FOR GAS DISCHARGE DEVICES Nov 24, 1953 Patented Nov. 24, 1953 CONTROL ELECTRODE-ANOD'E' STRUCTURE FQRr GAS DISCHARGE DEVIGES Edward 0.- Johnson, Princeton, N. .L, assignoig to Radio Corporation, of. America, a. corpdra;

tion of. Delaware Application October '23, 1952, Serial:No,..31.6,4.52;

8.Claims.. l.

Thisinvention relates tozimprovements in gas discharge devices that have continuous gridicontrol; More particularly; it: relatesv to.- improvements in gas discharge devices. of: a. particular kind. having. very high. values of? transconductrance and. of; anode;- current with very low values of output impedance. Devices of this general nature. are described more completely in an article by 0. Johnson andW. Webster in the Proceedings of theI. R. E; of June 1952;

The type of gasdischarge device with which my invention is concerned, normally consists of an anode, two cathodes, an apertureds focusing electrode, and a control: electrode. The cathodes are usually referred toas a main cathode and an auxiliary cathode. The auxiliary cathode is normally surrounded by the apertured focusing electrode; The control eleotrode-is'norm-ally interposed between the main cathode and the anode.

In operation of this type of discharge device, or tube, some of the electrons emitted-bytheauxiliary cathode pass through the apertured focusing electrode into the region between the main cathode and main anode where they ionize the gas molecules resulting in the production of a plasma. The plasma consists of a region of very high but essentially equal concentrations of ions and free electrons which due to the high mobility of the plasma electrons, and the absence of a, net space charge, operates as an extremely low impedance path for the load current in the tube. The apertured focusing electrode may be used to modulate the plasma entering the region between main cathode and anode. The control electrode, in the main cathode-main anode path modulates the electron flow from the main cathode to the anode and thus the output current of the device.

In devices of this kind, there are usually two separate discharge paths: one for the load current, and the other for an ionizing current. The energizing potential required for drawing the load current from the main cathode to the anode is set at well below the value of potential required to produce ionization so that, under normal operation, the load current can not ionize the gaseous filling of the tube. A higher potential is used to produce a separate ionization, or auxiliary discharge, within the tube. This auxiliary discharge, which operates independently of the work or load current, ionizes the tubes gaseous filling thereby forming a conductive plasma for the load current path of the tube.

plas ni; ssentiallr unafi ted y n ec cdeoatential. If an electrode is negative with re.- pca o pla ma. n a ive e rons ar nclled leas n only ei nof p sitiye, i ns en.- route cethe lcc rode- Such a. r ion iscalled: a positive ion sheath and will normally surround a. c nic-Q11 electrode. since the contr l; e trod w ll runs-I1ialhgbe: negative with. respect: to, the plasma during operation or the device. If an electrode. s liil ltivewith respect to, the plasma, the opposite; e fect will; take place and an electron s eath will surround the electrode.

The. output. current; of the device may be continuouslycontrolled or modulated by the. control electrode. This. occurs because the thick-- mess, on radius, of' the positive ion sheath. will vary in response. to a. variation of voltage on the control electrode. When the outputourrent is completely out. off the adjacentshea-ths. overlap. Intermediateoutput currents result from sheaths .of intermediate radii.

The action of a negative control electrode generally consists of twov effects, i. e. a more negative potential on the control electrode (1) decreases the effective area of the anode and (2) decreases the density of the plasma that is adjacent the effective area of the anode. The first effect is an extremely iast action and is thus desirable for high frequency operation. However, the second effect, i.e. decreasingthe density of the plasma is a relatively slow action because both negative electrons and positive ions are gated by the control electrode. Electron gating is the modulation of electron flow through the apertures of the control electrode and due to the high mobility of the electron particles is an extremely fast action. Positiveion gating is the modulation of positive ion flow through the control electrode wires and due to the relatively low mobility of the positive ion particles is a comparatively slow action. The gating action of both types of particles occurs because the control electrode is surrounded by the plasma containing both types of particles which results in the low resistance path between the cathode and anode.

Positive ion gating in this type of device results in poor high frequency response,- and linearity. The poor high frequency response occurs primarily because of the low mobility of the positive ions. In other words when the radii of th control electrode sheaths are such as to overlap, the source of plasma will be cut off from the control electrode-anode region. However, a certain amount of plasma will remain in the control electrode-anode region until the electrons difiuse out to the anode and the positive ions diifuse out to the control electrode. Because of the nature of the plasma, as a general rule, no electron can diffuse out of this region until a positive ion also diffuses out. This later phenomenon is true because of the great attractive force that the positive ions have for the electrons. In other words a field cannot be applied to the anode, under normal operating conditions, that will be sufficient to disrupt the plasma.

The devices of this type that have been constructed prior to this time have also had poor linearity. This occurs because the density of the plasma in the control electrode-anode region does not respond completely, until after a certain amount of diffusion occurs, in response to a change in potential on the control electrode. This phenomenon is due to the plasma that remains in the control electrode-anode region after a change in potential has been applied to the control electrode.

It is therefore an object of this invention to provide a new and improved gas discharge device, of the continuous control type having improved characteristics.

It is a further object of this invention to provide a new and novel gas discharge device with improved frequency response.

It is a still further object of this invention to provide a new and novel gas discharge device that has an improved linear output characteristic.

A still further object of this invention is to provide a gas discharge device having a new and improved control electrode action.

These and other objects are attained in accordance with the general aspects of this invention by providing a gas discharge device comprising an auxiliary cathode enclosed by an apertured focusing electrode, a main cathode, a control electrode, and an anode wherein the control electrode and anode are coplanar.

These and other features and advantages will best be understood from the following descriptions of the illustrated embodiments when read in connection with the accompanying drawings wherein like reference characters designate similar parts throughout the several views and in which:

Figure 1 is a schematic diagram of a prior art tube illustrated to point out the theory of operation of this type of structure;

Figure 2 is a schematic diagram of an improved type of tube made in accordance with this invention;

Figure 3 is a longitudinal sectional view of a gas discharge device constructed in accordance with this invention; and

Figure 4 is a transverse sectional view therefore through line 33 of Figure 3.

In Figure 1 there is shown a schematic diagram of the prior art type of tube that generally comprises an auxiliary cathode l2, surrounded by the apertured focusing electrode M, a main cathode IS, a plurality of control electrodes l8 and an anode 20. In operation of the device, the ionization is caused by a difference of potential between auxiliary cathode l2 and the main electrodes, i. e. main cathode l6, control electrode l8, and anode 26.

The auxiliary discharge is focused in the direction of the main cathode 15 by means of apertured focusing electrode Hi. When the auxiliary discharge occurs between auxiliary cathode i2 and the main electrodes, the device is filled with a plasma II. A characteristic of this type of device is the sheath that surrounds each of the electrodes in the device. The sheaths that are considered in this invention are the control electrode sheath and the anode sheath. Surrounding the control electrode l8 will be a positive ion sheath ii and adjacent the anode as will be the negative electron sheath 19. The plasma will diffuse through control electrode 58 into the control electrode-anode region. When a more negative potential is applied to the control electrode IS, the action of the control electrode will be such as to (1) decrease in the effective area of the anode, i. e. the anode area. directly behind the control electrode wire will be made inactive except for some slight diffusion in this area which may be neglected (the effective volume of plasma H is shown as a dotted line :3 in the control grid-anode area) and (2) the density of the plasma in the control electrode-anode area will be decreased, i. e. a smaller number of plasma particles will be permitted to pass through the control electrode thus reducing the plasma density in this region.

Assuming that it is desired to completely out off on electron flow to the anode 28, a potential is applied to the control electrode i8 which will cause the radii of the control electrode sheaths I! to overlap thus eliminating electron flow from the main cathode It to anode 26. Once this is done, none of the anode area will be directly affected by electron flow from the main cathode l6. However, there will remain in the control electrode-anode area a plasma. This plasma will remain until the electrons and positive ions have diffused out. Due to the electrode potentials, the electrons must diffuse to the anode 25, while the positive ions diffuse to the control electrode l8. On an average, due to the nature of the plasma, it is impossible for the highly mobile electrons to diffuse out until the comparatively slow ions have also diifused. It is because of this that this type of discharge device has relatively poor frequency response. In other words, the plasma remaining in the control electrode-anode region will remain for a time after the path has been cut off.

The second effect of the control process of the control electrode 4 is that the density of the plasma that is adjacent the effective area of the anode changes in response to a change in potential on the control electrode l8, This is true because of the effect of the electron flow to the anode, that is when a potential is applied to the control electrode 18, assuming for the moment that it Will not completely out off the flow to the anode 29, a particular number of plasma particles will be permitted to pass through the control electrode 18. Since the number of plasma particles that pass through the control electrode I8 will be less than the number of plasma particles that were passing through before the negative potential was applied, the density of the plasma adjacent anode 20 will be decreased. However, a plasma of the original density, before the change of potential on the control electrode l8, i. e.'a residual plasma, will remain in the control electrode-anode region until the plasma has diffused to the electrodes and the new density of the plasma is established. This residual plasma density results in poor linearity of the device.

In Figure 2, there is shown a schematic diagram of a gaseous discharge device that is constructed in accordance with this invention that will eliminate these disadvantages by placing the control electrode 28 and anode 30 in a single plane. When this arrangement is utilized the second effect of the control process of the control electrode will be el minated, i. e. the change in plasma density. This occurs because of the fact the control electrode positive ion sheaths 21 will overlap the anode and its electron sheath 29, when the device is completely out on. Further when it is desired only to modulate the electron flow, the control electrode 28 does not permit a plasma of an original density to remain adjacent the anode 3!] when the modulating potential is applied to the control electrode 28. This occurs because of the fact that there is ab solutely no space available in which the plasma 22' can exist after the potential is applied to the control electrode 28.

Referring now to Figures 3 and 4, I have shown one embodiment of my invention comprising an envelope 8 containing an ionizable medium. Any suitable gas or mixture of gases may be utilized. The gas pressure will vary in acccordance with the specific envelope and electrode geometry and spacings and is not believed to be critical. In the tube now being described helium at a pressure of approximately 750 microns was used though other gases and pressures may be used. The device is provided with the usual stem 26 through which the lead in conductors are sealed in the usual manner. The electrode assembly is supported by means of the conductors within the envelope as indicated. Between upper and lower insulating members 40 and 4| are mounted a main cathode 26, a control electrode or grid 28 and an anode 39 comprising rodlike members. Cathode 26 is the usual oxide coated sleeve type cathode having a conventional indirect heater extending therein, the leads of which are connected to supporting conductors in the usual manner. The upper end of the main cathode 26 is crimped in the well known manner to lock the same in place between the insulating members.

Control electrode or grid 28 comprises a plurality of parallel spaced wires, or rods, which extend into holes or recesses formed by the insulating members 49 and 4|. The control electrode is energized by the usual lead-in 23. The

control electrode wires, or rods, extend through the upper insulating member 49 and are there interconnected by means of wires 2!, or they may be interconnected by straps or other conventional means.

The anode 30 comprises a plurality of parallel spaced wires which extend into holes or recesses formed in insulating members 40 and 4|. The anode wires extend through mica 4! where they are interconnected by means of wires 25 or by other conventional means. The anode wires 30 are each intermediate one of the control electrode wires 28. Adjacent the open end of the structure formed by the control electrode 23 and anode 39, and near main cathode 26 is mounted a hollow cylindrical focusing electrode 24 supported between insulating members 40 and 4| by means of wires or support rods 33. Auxiliary cathode 34 is conveniently mounted concentric with the focusing electrode 24; the latter having a slot or 6 elongated opening 35 formedtherem opening'toward the main cathode 26. Auxiliary cathode 34 maybethe usual oxide coated thermionic type of cathode, and" is supported between insulating members 40 and 4t in-the conventional manner.

It should be noted that the alternate wires in the control electrode and anode structure are control electrode wires 28. This allows a control electrode 28 to be adjacent the apertured focusing electrode 24 and therefore permits effective control action. The anode 30 wires or rods are intermediate the control electrode wires 28 and are in the same plane. The control electrode 28 and anode 30 are mounted in the same plane so that the sheath from the control electrodes 28" will overlap the anode 30 and will completely engulf the anode structure instantaneously when it is desired to cut oif on electron flow to the anode.

The coplanar relationship existing between control electrode 28 and anode 30 eliminates any change in plasma density, adjacent the anode, caused by the gating action of the control electrode 28. The reason for this is that the sheath surrounding the control electrode wires will spread out to decrease the effective area of the anode when control electrode gating occurs and will not permit any area to remain adjacent the anode where the control electrode is not effective.

The spacing and size of wire used in the coplanar control electrode 28 anode 30 structure is not critical. One structure that has been found to operate satisfactorily is a structure using wire of approximately 10 or 20 mil diameter with a spacing between adjacent wires of approximately 1 mm. This particular structure is not intended to be limiting but is merely given as an efiicient type of structure. If desired the control electrode wires 28 and the anode wires 30 may be of different diameter and if so it is preferred to have the control electrode wires the larger of the two in order to have quicker control electrode action.

I claim:

1. A gas discharge device, comprising a sealed envelope, an ionizable medium in said envelope, a thermionic cathode, a control electrode and an anode mounted in the same plane in said envelope, said control electrode and said anode being spaced apart within a range where a sheath surrounding said control electrode afiects a sheath surrounding said anode, and means in said envelope for producing a plasma, said plasma normally extending continuously without interruption from adjacent said cathode to adjacent said anode.

2. A gas discharge device, comprising a sealed envelope having an ionizable medium therein, a thermionic cathode, a control electrode spaced from said cathode, an anode spaced from said cathode, said control electrode and said anode being coplanar, said control electrode and said anode being spaced apart within a range where a sheath surrounding said control electrode affects a sheath surrounding said anode, and means in said envelope for producing a plasma, said plasma normally extending continuously without interruption from adjacent said cathode to adjacent said anode.

3. A gas discharge device, comprising a sealed envelope having an ionizable medium therein, a thermionic cathode, a plurality of control electrode conductors spaced from said cathode, a plurality of anode conductors spaced from said cathode, said control electrode conductors and said anode conductors being spaced apart within a range where a sheath surroundingsaid control electrode afiects a sheath surrounding said anode, said control electrode conductorsand said anode conductors being coplanar, and means in said envelope for producing a plasma.

, 4. A gas discharge device, comprising a sealed envelope containing an ionizable medium, a thermionic cathode, a plurality of control electrode conductors spaced from said cathode, a plurality of anode conductors spaced from said cathode, each of said anode conductors being intermediate said control electrode conductors, said anode conductors and said control electrode conductors being coplanar, and means in said envelope for producing a plasma.

5. A gas discharge device, comprising a sealed envelope containing an ionizable medium, a thermionic cathode, a unitary control electrodeanode means spaced from said cathode and in said envelope, said means comprising alternate. spaced apart control electrode conductors and anode conductors, all of said conductors being coplanar, said alternate conductors being insulated from said intermediate conductors, and an auxiliary thermionic cathode.

6. A gas discharge device, comprising a sealed envelope containing an ionizable medium, a thermionic cathode, a plurality of spaced apart conductors spaced from said cathode and Within said envelope, the alternate of said conductors being control electrode conductors, the intermediate of said conductors being anode conductors, all of said conductors being coplanar, and means in said envelope for producing a plasma.

7. A gas discharge device, comprising a sealed envelope having an ionizable medium therein, a pair of insulating members in said envelope, a thermionic cathode extending between said insulating members and supported thereby, a plurality of spaced apart rods extending between said insulating members and spaced from said cathode, said rods being coplanar, the alternate of said rods being control electrode rods and extending through one of said insulating members and being interconnected, the intermediate of said rods being anode rods and extending through the other of said insulating members and being interconnected, and means in said envelope for producing a plasma.

8. A gas discharge device, comprising a sealed envelope having an ionizable medium therein, a pair of spaced parallel insulating members in said envelope, a thermionic cathode extending between said insulating members and supported thereby, a plurality of spaced apart parallel rods extending between said insulating members and spaced from said cathode, said rods being coplanar, alternate rods being control electrode rods and being electrically connected, the intermediate rods being anode electrode rods and electrically connected, an auxiliary thermionic cathode spaced from said electrodes, and a focusing electrode adjacent said auxiliary cathode, said focusing electrode being adapted to direct electrons toward said main cathode to form a plasma.

EDWARD O. JOHNSON.

No references cited. 

